The present invention relates to selectively plugged honeycomb structures, and in particular, to a method and apparatus for use in selectively plugging cells of a honeycomb structure for the fabrication of filter bodies.
Honeycomb structures having transverse cross-sectional cellular densities ranging between 10 and 300 cells/in2 (about 1.5 to 46.5 cells/cm2), more typically between about 100 and 200 cells/in2 (about 15.5 to 31 cells/cm2) and wall thicknesses of between 0.010 and 0.030 in. (about 0.25 to 0.76 mm), especially when formed from ceramic materials are used as solid particulate filter bodies, and require selected cell channels of the structure to be closed or plugged at one or both of their ends.
It is well known that a solid particulate filter body, such as a diesel particulate filter, may be formed by a matrix of intersecting, thin, porous walls which extend across and between two of its opposing end faces and form a large number of adjoining hollow passages or cell channels which also extend between and are open at the end faces of the structure. To form a filter, one end of each of the cell channels is closed, a first subset of cell channels being closed at one end face and the remaining cell channels being closed at the remaining opposing end face of the structure. Either of the end faces may be used as the inlet face of the resulting filter. The contaminated fluid is brought under pressure to the inlet face and enters the body via those cells which have an open end at the inlet face. Because these cell channels are closed at the outlet end face of the body, the contaminated fluid is forced through the thin, porous walls into adjoining cells which are sealed at the inlet face and open at the outlet face of the filter body. The solid particulate contaminant in the fluid which is too large to pass through the porous openings in the walls is left behind and a cleansed fluid exits the filter body through the outlet cell channels, for use.
Up to this time selected cells were sealed or plugged with a foam-type cement, as disclosed in U.S. Pat. Nos. 4,297,140, 4,293,357 and 4,559,193. The cement is formed into a paste by mixing ceramic raw material with an aqueous binder, such as methyl cellulose, plasticizer and water. When using this foam-type cement, both ends of the honeycomb structure are covered with flexible masks having holes through which the cement is pushed into the ends of the cells. There are numerous disadvantages associated with this type of filling or plugging material. Masks are applied manually to the honeycomb ends, must be cleaned and dried after each use. Unclean masks can cause missing plugs requiring additional manual labor. The cement batch is time, shear and temperature dependent; often thrown out, unused due to age restrictions; and, drying is required to remove the water content.
The process of the invention is provided to overcome the disadvantages of prior art plugging methods. The process is for plugging with a powdered plugging material, a selected subset of cell channels of a ceramic honeycomb structure having a multiplicity of mutually adjoining cell channels which have open ends at and extend throughout the structure between opposing end faces. xe2x80x9cPowdered plugging materialxe2x80x9d as used in the description of the present invention means a material which is formed of particulates. This type of plugging material is in contrast to current prior art plugging materials which are a cement, having a wet, paste-like consistency. Although any powdered plugging material suitable for forming plugs may be used in the present invention, the preferred material comprises particulate sealant as described and claimed in co-pending patent application filed concurrently with the present application in the names of by M. Fabian, S. Lakhwani, and M. Roberts under the title PARTICULATE SEALANT FOR FILTER PLUG FORMING.
In practice, powdered plugging material is loaded through the open end portions of all the cell channels at one end face of the honeycomb structure and passes through the structure settling at the opposing cell end portions. Next, the powdered plugging material is compacted by suitable compaction means, such as vibration, centrifuge, manual compaction and the like. An optical image analyzer, which includes a camera, is used to scan and locate a selected subset of cell channels which are to be plugged. In response thereto, a first set of signals is generated indicating the location the subset of selected cell channels, the signals being inputted to a microprocessor. A second set of signals is then generated by the processor to position a laser with respect to the selected subset of cell channels, whereby the laser system applies laser energy to the powdered plugging material to form plugs. In the application of laser energy, the powdered plugging material is heated sufficiently to become flowable, adhering to the cell walls. Upon cooling the material re-solidifies and a plug is formed at the end portions of the selected cell channels. The remaining plugging material which has not been exposed to the laser energy is easily uncompacted and then removed from honeycomb structure. The so-plugged honeycomb structure is fired to sinter the plugs and form the desired final structure.
In one embodiment, a solid particulate filter is fabricated by forming plugs in a first subset of cell channels of a honeycomb structure at one of its end faces; removing the plugging material in a remaining second subset of cell channels by uncompacting the plugging material from the end portions thereat; and, reusing the uncompacted plugging material to form plugs at the remaining end portion of the second subset of cell channels. Therein results a honeycomb structure having all of the cell channels plugged at only one end portion, preferably in an alternate checkered pattern.
In a second embodiment, a solid particulate filter body is formed by forming plugs in a first subset of selected cell channels of a honeycomb structure at one of its end faces; removing the plugging material in the remaining second subset of cell channels; loading a new batch of plugging material into the honeycomb structure through the open ends of the second subset of cell channels at the plugged end face; passing plugging material thorough the second set of cell channels to the opposing end portions, where it is compacted and exposed to laser energy to form adherently bonded plugs. Therein results a honeycomb structure having all of the cell channels plugged at only one end portion, preferably in an alternate checkered pattern.